A. Field of the Invention
This invention relates to a manual jack for moving a linear actuator stem of a motor valve in normal operation from a closed position to an open position, but which, when exposed to temperatures above a certain level, will automatically allow the stem to move to the closed position.
B. Description of the Prior Art
Many gate valves are equipped with hydraulic or pneumatic linear actuators that are adapted to move the valve between open and closed positions. Gate valves so equipped are commonly referred to as motor valves. Typically, hydraulic or pneumatic pressure in the actuators holds the gate in the open position. Upon release of the pressure within the actuator, pressure within the valve body acting upon the valve stem moves the gate to the closed position. Normally, the operation of motor valves is automatic.
In many instances, however, it is desirable to override the operation of the linear actuator and manually move the valve the open position. Manual override may be accomplished by mounting a jacking apparatus to the actuator. The jacking apparatus is used to move the stem to the open position, where it will remain until the jacking apparatus is operated to close the valve.
However, prudent operations and some government regulations require that an unattended valve have means for allowing the valve to close in case of fire.
One heat sensitive manual override device of the prior art is disclosed in Wicke, U.S. Pat. No. 3,842,854. In the device of Wicke, the jacking apparatus is releasably connected to the actuator body. A fusible element operates to maintain the connection between the jacking apparatus and the actuator body. Upon melting of the fusible element, the jacking apparatus and actuator body disengage, whereupon pressure within the valve body drives the gate to the closed position. It isintended that after disengagement, the jacking apparatus of Wicke will simply fall off the actuator. In actual practice, however, the line pressure acting across the valve stem when the operator becomes disconnected causes abrupt and forceful movement, which converts the operator into a projectile. The operator may be propelled several tens of feet before it comes to rest, which creates an obvious hazard.
It is therefore an object of the present invention to provide a heat sensitive operator that is operable to move a linear actuator stem between open and closed positions during normal operations and, upon being exposed to excessive temperatures, being operable to automatically allow the stem to move to the outer position.
Another object is to provide such an operator which, upon being exposed to excessive temperature, permits the stem to move under the influence of line pressure to an outer position without producing any pieces or mechanisms which could act as projectiles and present a hazard to nearby personnel and equipment.
Another object is to provide such an operator wherein the fusible element is arranged in such a manner as to dampen the movement of the stem and prevent slamming and abrupt impacts in the actuator or valve.
Another object is to provide such an operator which is constructed so that after melting of the fusible element and movement of the stem to its outer position, it will be readily apparent to an observer that the stem is in such position.